1. FIELD OF THE INVENTION
This invention relates to optical interference filters made of alternating layers of tantala and silica and their use on lamps. More particularly, this invention relates to optical interference filters made of alternating layers of tantala and silica in which subsequent extrinsic stress in the tantala layer due to crystallization is reduced by doping the tantala with titania and/or contacting the tantala layer with a layer of titania, and the use of such filters on electric lamps.
2. BACKGROUND OF THE ART
Thin film optical coatings known as interference filters which comprise alternating layers of two or more materials of different indices of refraction are well known to those skilled in the art. Such optical coatings are used to selectively reflect or transmit light radiation from various portions of the electromagnetic spectrum and are used in the lamp industry to coat reflectors and lamp envelopes.
Interference filters to be used for applications where the filters will be exposed to high temperatures in excess of about 500.degree. C. have been made of alternating layers of tantala (tantalum pentoxide, Ta.sub.2 O.sub.5) and silica (SiO.sub.2), wherein the silica is the low refractive index material and the tantala is the high refractive index material. Such filters and lamps employing same are disclosed, for example, in U.S. Pat. Nos. 4,949,005; 4,689,519; 4,663,557; and 4,588,923, the disclosures of which are herein incorporated by reference.
The problem of severe stress formation associated with the use of interference filters consisting of alternating layers of silica and tantala when employed at high temperatures was recognized in U.S. Pat. No. 4,734,614, the disclosure of which is herein incorporated by reference. This patent teaches that tantala has limited physical and chemical stability, and crystallizes to a polycrystalline form after about 30 minutes at 800.degree. C. which produces stress cracks visible as craquele. This results in a filter that scatters both visible and infrared radiation, thereby rendering it unsuitable for its intended purpose.
Responsive to the need for a process which would apply a relatively uniform coating to a complex shape and which would not result in a film containing such stresses that the film cracks and peels off the substrate, U.S. Pat. No. 4,949,005 to coinventor herein, T. Parham et al., provides an interference filter thin film optical coating consisting essentially of alternating layers of tantala and silica having relatively low light scattering which is suitable for use at high temperatures. The coatings are obtained by employing a chemical vapor deposition (CVD) process and, preferably, a low pressure chemical vapor deposition (LPCVD) process to produce the coatings on a suitable substrate, such as quartz. After application of the alternating layers, the optical filter produced is annealed to avoid catastrophic extrinsic stress formation from volume changes due to crystallization of tantala at temperatures above about 600.degree. C. which cause cracking and buckling, and result in poor adhesion, peeling, and undesirable optical scattering of light. Thus, the coated substrate must be heated to a temperature of between about 550.degree. to 675.degree. C., and held within this temperature range for a period of from about 0.5 to 5 hours.
Despite advances in the art, however, the need to anneal subsequent to deposition in order to reduce stress continues to be mandatory and increases the cost and time required for manufacturing interference filters and articles, such as lamps, including these interference filters.
Accordingly, it is an object of the present invention to provide a tantala-silica optical interference filter in which the extrinsic stress of the tantala layer is reduced so that the need for post-deposition annealing is reduced and, when combined with an annealing treatment, for example, to relieve stress in the silica layers, produces filters having reduced stress-related damage.
It is another object of the present invention to provide a low stress tantala-silica optical interference filter produced by a CVD or LPCVD process.
It is yet another object of the present invention to provide a lamp including a low stress tantala-silica optical interference filter so that the energy efficiency of the lamp is improved.